This application claims the priority of German patent document 197 12 728.2, filed Mar. 26, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a piezoelectric loudspeaker having a membrane and a flexural piezo resonator.
In conventional piezoelectric loudspeakers, an individual flexural piezoresonator is securely mounted on one end, and its free end acts on a cardboard membrane, deflecting it. A flexural resonator of this type is described, for example, in Brockhaus, "Natural Science and Technology", Mannheim, Special Edition, 1989, Volume 3, Page 159. Since, in this case, partial vibrations and deformations of the membrane are undesirable, the membrane must be as stiff as possible. For this purpose normally a steep conical shape is selected, which requires a large height, considerably restricting the possibility of placement, for example, in an automobile.
It is therefore an object of the invention to provide a piezoelectric loudspeaker having an extremely low height.
This and other objects and advantages are achieved by the loudspeaker according to the invention, in which several flexural piezoresonators that are used as the drive are fixed on one end, and can be constructed to be extremely flat. The free end of these elements act upon a flat membrane or a conical membrane, the cone having a very flat construction (base angle less than 10.degree.). The position of the individual flexural piezoresonators with respect to the membrane can be freely selected.
By means of the construction according to the invention, heights can be achieved which are lower than approximately 10 mm. Despite this flat construction, the generation of partial vibrations of the membrane can be avoided.
It is another advantage of the invention that the individual flexural piezoresonators can be operated with different phase positions relative to one another, thereby considerably improving the frequency response of the loudspeaker.
Since the natural frequency of the flexural piezoresonator depends on its free length, breaks in the frequency response can be compensated by adjusting different free bending lengths for the individual flexural piezoresonators. A combination of flexural piezoresonators with different free lengths causes these natural frequencies to occur over a wide frequency range. Thus, skillful adaptation provides a simple possibility for tuning the frequency response of the loudspeaker.
In a particularly advantageous embodiment, a flexural piezoresonator consists of several cascaded flexural piezoresonators. As a result, the stroke or the force exercised on the membrane can be increased.
Advantageously, a carbon fibre kevlar (CFK) sandwich membrane with a kevlar insert between the sandwich core and the cover layer is used as the membrane. This CFK sandwich membrane may be constructed as a flat membrane as well as a flat cone, and has the advantage of a very high stiffness.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.